Aromatic polycarboxylic acids are commercially important substances as chemical intermediates. Thus, there is a wide demand for aromatic polycarboxylic acids as raw materials of polyesters, polyamides, polyimides, liquid crystal polymers, etc. which are used for fibers, bottles, films and electronic applications.
As currently widely industrially used aromatic polycarboxylic acids, there may be mentioned terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, pyromellitic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid and 3,3′,4,4′-biphenyltetracarboxylic acid.
Known methods for the preparation of an aromatic polycarboxylic acid include a method in which a polyalkyl aromatic hydrocarbon such as xylene, dialkylnaphthalene, dialkylbiphenyl, tetraalkylnaphthalene or tetraalkylbiphenyl is oxidized with molecular oxygen at a high temperature and a high pressure in the presence of a heavy metal such as Co or Mn and a bromine compound in an acetic acid solvent, and a method in which the polyalkyl aromatic hydrocarbon is oxidized with air in the presence of nitric acid, chromic acid or the like. The aromatic polycarboxylic acid obtained by the above oxidation reaction contains impurities such as monocarboxylic acids and aldehydes which are intermediate products of the oxidation reaction, bromine adducts and metal components which are derived from the catalyst and coloring substances having unknown structures.
As a recent increase of necessity for recycling plastic materials such as polyesters, materials are now recycled and reused through, for example, decomposition of PET bottles. In general, however, aromatic polycarboxylic acids obtained by the above decomposition contain impurities such as colored substances and foreign matters.
When the aromatic polycarboxylic acids containing such impurities are used as raw materials for the polymerization with diols or diamines, physical and mechanical properties, such as heat resistance, mechanical strengths and dimensional stability, of the obtained resins are inferior. Therefore, such aromatic polycarboxylic acids cannot be used as raw materials for polyesters, polyamides and polyimides. Further, crude aromatic polycarboxylic acids obtained by oxidation are generally colored yellow or black and cannot be used as such for applications requiring transparency such as bottles and films.
In this circumstance, as a method of purifying terephthalic acid, for example, a method is widely used in which a crude terephthalic acid is completely dissolved in water as a solvent at a high temperature of 260 to 280° C. The solution is then subjected to hydrogenation using a palladium catalyst supported on activated carbon so that impurities such as polymerization inhibitory substances and substances causing coloration are reduced. From the resulting solution, terephthalic acid is crystallized. By this method, purified terephthalic acid capable of being directly used as such for polymerization may be obtained (Japanese Patent Publication No. 41-16860).
The above method is for terephthalic acid which is easily soluble in water at a high temperature. In order to improve productivity, however, it is necessary to use a temperature as high as 260 to 280° C. and, accordingly, to use a high pressure. Because such a high temperature is used, side reactions such as hydrogenation on the nucleus are apt to occur and, further, it is necessary to select materials of the apparatus while taking corrosion thereof into consideration.
In the case of naphthalenedicarboxylic acid and biphenyldicarboxylic acid, since the solubility thereof in water is about 1/10 of that of terephthalic acid, it is necessary to use much higher temperature than 280° C. in order to conduct the above purification method with high productivity. This causes extreme difficulty in practical use.
Purification of an organic compound is generally performed by distillation, crystallization, adsorption or a combination of these operations. Since aromatic polycarboxylic acids have a self-decomposition temperature which is lower than the boiling point thereof, the purification by distillation is substantially impossible. Further, since aromatic polycarboxylic acids have poor solubility in commonly industrially used solvents, the purification by crystallization involves difficulties. In particular, since naphthalenepolycarboxylic acid and biphenylpolycarboxylic acid are hardly soluble in various solvents, industrially advantageous processes for producing high purity naphthalenepolycarboxylic acid or high purity biphenylpolycarboxylic acid have not yet been established.